The tectum of the midbran is an important multisensory integrating center subserving arousal and orientation behavior in vertebrates, yet little is known of the details of its intrinsic synaptic organization relating to input from specific sensory sources. A wealth of electrophysiological data are available concerning the occurrence of multimodal input to the tectum, but the response characteristics of units receiving concurrent multimodal stimuli have been described in only a few cases. One system in which multisensory interaction and responses have been studied extensively is the trigeminal infrared receptor-visual interaction in the tectum of crotalid and boid snakes. In these as in other species the retinal projections end in the superficial layers with some penetration into deeper laminae, while the non-visual, i.e., infrared projections are juxtaposed in the deeper layers. Biomodal units are located in the intermediate laminae. However, in this as in other systems little is known of the details of the neuronal interconnections of the specific sensory inputs. The invstigation is designed to determine the synaptic substrate for biomodal interaction in the optic tectum of an infrared sensitive snake. The study will focus on the intermediate laminae: stratum griseum centrale and stratum fibrosum centrale. Golgi preparations will be studied with an image-combining computer microscope and a light microscope with drawing tube to obtain quantitative and qualitative data on the morphology and dendritic architecture of the neurons in these laminae. These Golgi impregnanted neurons will also be examined by electron microscopy to determine the morphology of synapses on the different cell types. Infrared projections to the tectum will be labelled by anterograde transport of HRP. Computer microscopy will be employed to study the distribution and architecture of the labelled axonal arbors. The synaptic relationships of the labelled terminals will be determined by electron microscopy. A combination of HRP labelling of infrared afferents and degeneration following enucleation will be used to localize terminals of each sensory projection to the tectum. The relationship of these projections to individual neurons in the tectum will be investigated by combining lesion-degeneration of each afferent projection with Golgi impregnation and examination at the EM level. These experiments will help to identify sysnapses in relation to specific sensory input in the rattlesnake tectum. Such information is fundamental to the understanding of tectal activity in this species and should be generally relevant as the reptilian tectum appears to have the same functional organization as other species including the superior colliculus of mammals. Data from the proposed studies will provide a foundation for understanding a component of behavior in terms of basic neuronal interconnections.